Chip Joyce eloquently makes the argument for homosexuals being legally allowed to marry:

Marriage has been defined as heterosexual only because of religion. Period.

It is obvious to honest people who don’t live in complete seclusion, that gay couples exist with the exact same relationship as married people have, and they need and deserve the same legal recognition and protection.

The definition of marriage has been wrong because of religious bigotry. The concept of marriage applies to gay couples too, and the definition needs to be corrected to accurately refer to the concept.

It’s like this: a bad definition of swans said they are white, and then there was a black one. The concept “swan” is the same, regardless, and the definition is demonstrably flawed. In the same way, there is no reason for “man and wife” to be in the definition of “marriage” any more than the color white be in the definition of “swan.” “Man and wife” and “white” are non-essential and should be omitted. Marriage is a type of relationship between two consenting adults.